1. Field Of The Invention
This invention relates to photoconductive devices such as solar cells for converting light into electrical energy and more particularly to thin film so1ar cell modules.
As used throughout this specification and the claims, the following terms have the following meanings:
"Solar Cell" or "cell", an individual discrete member having a junction therein and capable of directly converting photons to electrical energy;
"Thin film solar cell", a solar cell fabricated from microcrystalline, amorphous, or compound semiconductors, or semiconductor material other than single crystalline semiconductor material deposited in situ upon a substrate;
"Panel", an array or group of solar cells interconnected to provide an output of electrical energy;
"Module", one or more panels confined within an appropriate housing and capable of being placed in long term service for production of electrical energy;
"Array", depending upon the context, a group of solar cells forming a panel or group of panels positioned to receive photons for direct conversion to electrical energy;
"Spectral response", sensitivity to a predetermined portion of the light spectrum less than the whole thereof.
2. The Prior Art
It has long been desirable to capture as much of the sun's spectrum as possible to convert it directly into electrical energy through the utilization of solar cells. Conventional single crystal solar cells appear to be rapidly approaching the ultimate intrinsic limits of their conversion efficiency. As a result, other types of solar cells are being constructed, such as those made from gallium arsenide and other similar materials. While such materials may have a higher efficiency of conversion than single crystal silicon, there is a limit to the ultimate efficiency which can be expected.
To increase the collection efficiency, consideration has been given to cascading solar cells, as is discussed in Salahm Bedair, Sunil B. Phatak, and John R. Hauser, IEEE Transactions on Electron Devices, ED-27, No. 4: 822-831 (April, 1980). As is therein disclosed, one of the approaches to improve efficiency makes use of two or more cells to more efficiently utilize the solar spectrum. A first approach utilizing such plurality of cells is that of spectrum splitting. That is, the solar spectrum is split into two or more parts by the use of filters and as a result a narrower band of photon energies is incident on each individual cell. As a consequence, each cell must respond to a narrower range of photon energies and each of the cells can then be optimized at a higher efficiency than can one single cell for the entire solar spectrum. One experimental apparatus used a silicon single crystal cell for the low energy photons and an aluminum gallium arsenide (AlGaAs) cell for the high energy photons.
Another approach is to connect two individual solar cells in optical and electrical series. In this approach, the wide bandgap cell is located above the narrow bandgap cell. The high energy photons are then absorbed in the wide bandgap top cell while the low energy photons, i.e., those below the band gap of the top cell, pass to the bottom cell for absorption. The cascaded cells were formed by utilizing an aluminum gallium arsenide/gallium arsenide monolithic cell structure with a heavily doped, very thin tunneling interface layer having a bandgap as large or larger than that of the top cell.
It will be readily recognized that the spectrum splitting concept requires mirrors, filters, and two distinct solar cells. In addition, two distinct packages housing those solar cells and the spectrum splitting device are required. Those skilled in the art will readily recognize that the utilization of such a concept for commercial applications is not cost effective as compared to the state of the art solar cells.
The cascading of solar cells by optical and electrical series connection through the utilization of a thin highly doped tunneling interface requires matching of short circuit currents in order to achieve proper operation. This matching of short circuit currents becomes impossible when the cell is exposed to ambient sunlight, simply because the frequencies of the ambient light on earth change throughout the day as the sun moves across the sky. Thus it will be recognized by those skilled in the art that short circuit current matching cannot be accomplished except for one frequency of the spectrum or one particular spectrum of incident light. Furthermore, if single crystal structures are to be used for the cascaded cells as disclosed in the prior art, the interface connections require lattice matching to obtain the appropriate tunneling through the interface. Such has proven to be ineffective.
In each of the instances in the prior art mentioned above, individual solar cells have been dealt with as opposed to interconnected arrays of such cells forming a complete panel of solar cells.
U.S. Pat. No. 4,461,922 to Charles F. Gay, V.K. Kapur, and James H. Wilson, the disclosure of which is incorporated herein by reference, describes a solar cell module in which individual panels of solar cells are stacked one on top of another and arranged so that incident light passes through each of the arrays of cells in each panel, thereby striking the one below it. In this patent, the solar cells in each panel are selected to have a predetermined and different spectral response and thus be responsive to different frequencies of light. According to the patentees, each of the panels may independently be constructed from thin film silicon hydrogen alloys, single crystal, or compound semiconductors. Although this patent describes stacking solar modules in order to improve the efficiency thereof, there has been a need to maximize the utilization of light energy and to thereby enhance the efficiency of such stacked modules.
It is accordingly an object of the present invention to provide a photoconductive device in which the utilization of light energy is maximized.
Another object of this invention is to enhance the efficiency of photoconductive devices.
Other objects and advantages of the present invention will become apparent from the following detailed description.